The present invention relates generally to an image display apparatus, and more particularly to a head- or face-mounted image display apparatus that may be mounted on an observer's head or face for use.
In order to allow an individual to enjoy images on a large screen, image display apparatus, especially head- or face-mounted image display apparatus have now been under extensive development.
For instance, JP-A's 7-333551 and 8-234137 disclose such image display apparatus comprising an ocular optical system for guiding to the eyeball of an observer an image displayed on an image display device built up of a liquid crystal display device (hereinafter LCD for short). This ocular optical system is designed as a decentered optical system comprising three optical surfaces while a space surrounded therewith is filled with a medium having a refractive index of at least 1. A light beam is incident from the liquid crystal display device on the decentered optical system in which the incident light is reflected at a first surface, and then internally reflected at a second surface of a concave mirror. Finally, the light leaves the decentered optical system now through the first surface, so that the image displayed on the image display device can be guided to the observer's eyeball without forming any intermediate image.
In this case, the decentered optical system is made up of three optical surfaces and two internal reflections occur in the decentered optical system. Besides, there are available various embodiments of decentered optical systems comprising two optical surfaces or four or more optical surfaces, in which at least one reflection occurs, as proposed by the applicant.
The apparatus set forth in JP-A's 7-333551 and 8-234137 are designed to be used with a transmission type image display device or LCD (liquid crystal display device). However, a reflection-type LCD device, too, can be used for an image display device for face-mounted image display equipment, as typically proposed in JP-A 7-72446. An optical system for such image display equipment is shown in FIG. 18. As shown, illumination light from a lamp light source 55 is converted into parallel beams through a collimating optical element 56, and a part (s-polarized light) of the beams is reflected by a polarized beam splitter 57 to illuminate the front face of a reflection-type LCD 58. An imaged reflected and modulated by the reflection-type LCD 58 is projected through a projecting optical element 59 onto a screen 52, so that the thus projected image can be magnified and presented through an ocular optical element 53 to an observer.
In addition to the reflection-type LCD, an image display device referred to as a DMD (digital micro-device), too, is proposed for the reflection type image display device. This is constructed as shown in FIG. 19. More specifically, FIG. 19(a) is a plan view of DMD and FIG. 19(b) is a perspective view of DMD elements. As shown, there are two-dimensionally arranged micro-mirrors 60 corresponding to pixels, wherein an addressed mirror 60' is tilted with respect to a diagonal line, thereby reflecting incident light from a certain direction on the mirror 60' in a different direction with respect to a non-tilting mirror to display a two-dimensional image. Each mirror 60 is supported by a pair of supporting posts 62 positioned at diagonal corners by means of hinges 63. Voltage is applied on one of a pair of electrodes 64 provided on a substrate 61 with the mirror 60 formed thereon, so that the mirror 60 can be rotated by electrostatic power with respect to the diagonal line between the hinges 63 (IEEE Spectrum, Vol. 30, No. 11, pp. 27-31).
An advantage in using a decentered optical system as an optical system for image display equipment is that the whole equipment can be slimmed down while high optical performance (angle of view, resolving power, etc.) is maintained to present bright images to an observer. However, only a transmission-type LCD has been designed for an image display device used with the decentered optical system. For this reason, the decentered optical systems set forth in JP-A's 7-333551 and 8-234137 are all merely constructed as a telecentric optical system with an entrance pupil positioned at infinity.
For the transmission-type LCD used as an image display device, it is required to make inter-pixel black matrices less discernible by use of a low-pass filter or the like, because the black matrices are noticeable due to a low pixel numerical aperture. On the contrary, the reflection-type LCD is less sensitive to such problems because the pixel numerical aperture can be increased. However, when images are magnified and observed using a decentration-free optical system, it is required to illuminate the reflection-type LCD using an optical element such as a light beam splitter. Especially when used with head- or face-mounted image display equipment, this device is contrary to demand for size and weight reductions, and offers a dark image problem as well.
Especially when the reflection-type LCD has an angle-of-field feature of changing the contrast of displayed images depending on the direction of view, it is required to make the side of the optical system located on the image display device substantially telecentric. With a prior art optical system wherein an illumination light beam is reflected onto the image display device using a half-coated mirror (a polarized beam splitter), however, it is required to ensure an extra space where the half-coated mirror is to be positioned. This in turn gives rise to a size problem.